I can sort of see it now, off in the distant haze...
*dude lights a grenade to throw...
Guys in room where 'nade is thrown..
"Hey a nade!"
"Open your mouths!"
*nom nom nom nom BB's* (or corn, peas, etc..)
I can understand the pressure balancing, but in an airsoft enviroment I wouldn't feel too safe considering somebody may throw a grenade then enter after it blows to clear the room you're in.